The present invention relates to black magnetic iron oxide particles which are suitably used as black coloring pigments for paints, resins, printing inks, etc., because of a good blackness thereof, and more particularly, to black magnetic iron oxide particles which are capable of providing toners having a high image density even under high-temperature and high-humidity conditions when used as black magnetic particles for magnetic toners, because they are excellent in electrical characteristics, moisture absorption and dispersibility.
Further, the present invention relates to a magnetic carrier for electrophotographic developer which exhibits a sufficient electric resistance value and a less voltage dependency of the electric resistance value, and is excellent in gradation of obtained images, as well as a two-component developer comprising a toner and the magnetic carrier for electrophotographic developer.
Magnetite particles are typical black pigments, and have been generally used for a long time as a colorant for paints, printing inks, cosmetics, rubber and resin compositions, etc.
In particular, the magnetite particles have been frequently used in one-component type magnetic toners in which composite particles prepared by mixing and dispersing black magnetic iron oxide particles such as magnetite particles in resins are employed as a developing material.
In recent years, with the tendency of a high printing speed and a high image quality of laser beam printers or digital copying machines as well as the development of apparatuses capable of being operated under various environmental conditions, it has been strongly required to enhance properties of magnetic toners as a developer, in particular, provide toners capable of exhibiting a good keeping property of image density even under high-temperature and high-humidity conditions.
In order to meet the above requirements for the magnetic toners, it is also strongly required that the black magnetic iron oxide particles used therein are further improved in properties thereof.
More specifically, in order to obtain toners which are excellent in environmental stability, in particular, keeping property of image density under high-temperature and high humidity conditions, the black magnetic iron oxide particles used therein are required to have not only more excellent electric characteristics such as a sufficient resistance value, but also a low moisture absorption and an excellent environmental stability as well as an excellent dispersibility.
The reason therefor is due to the fact that upon forming a toner image, an image force as a resultant force of an electrostatic attraction force and a magnetic constraint force is exerted on toner particles when the toner particles fly towards a latent image formed on a photosensitive member, and an intensity of the image force is delicately controlled to attain a good image density. Namely, the toner particles having a high resistance value are improved in charging performance and, therefore, tend to readily fly towards the photosensitive member, resulting in a high image density.
In order to control the charging performance of the toner particles, there may be usually used a charge controlling agent. As the other means for controlling the charging performance of the toner particles, there is known the method of controlling an electric resistance value of magnetic iron oxide particles as a pigment component exposed to the surface of the respective toner particles. More specifically, when the electric resistance value of the magnetic iron oxide particles exposed to the surface of the respective toner particles is high, the toner particles tend to be readily charged. On the contrary, when the electric resistance value of the magnetic iron oxide particles exposed to the surface of the respective toner particles is low, an electrostatic charge on the surface of the charged toner particles is leaked through the magnetic iron oxide particles exposed to the surface of the respective toner particles upon stirring in a toner hopper, resulting in reduction in charge amount of the toner particles.
These phenomena tend to become more remarkable under some environmental atmospheres to which the developing device is exposed, in particular, under high-temperature and high-humidity conditions. More specifically, in general, the charging performance of the toner tends to be lowered under high-temperature and high-humidity conditions, resulting in low image density.
Therefore, when the black magnetic iron oxide particles are used as a pigment for the toner particles, it is very important to well control electric characteristics and moisture absorption of the black magnetic iron oxide particles in order to obtain images having a high image density.
As to the electric resistance value of the black magnetic iron oxide particles, it is generally known that since magnetite exhibits electric characteristics of a semiconductor, a high electric resistance value thereof is realized by coating or attaching a high-resistance component (such as high-resistance oxides, hydroxides, dielectric organic substances, hydrophobic organic substances, etc.) on the surface of the respective black magnetic iron oxide particles by a dry or wet method.
Conventionally, it has been attempted to improve various properties of the black magnetic iron oxide particles by incorporating different kinds of elements other than iron thereinto and coating the surface thereof with an inorganic or organic substance.
For example, in Japanese Patent Application Laid-open (KOKAI) No. 2002-72545, there are described iron oxide particles comprising composite iron oxide of aluminum and iron on a surface thereof. Also, in Japanese Patent Application Laid-Open (KOKAI) No. 2005-289673, there are described magnetite particles obtained by subjecting magnetite particles having a coating layer comprising a compound of one or more elements other than iron to mechanochemical treatment.
Further, in Japanese Patent Application Laid-Open (KOKAI) No. 2007-314412, there is described a black magnetic iron oxide whose surface is coated with a surface layer comprising a compound of at least one alkali earth element and an Al element.
In addition, in Japanese Patent Application Laid-Open (KOKAI) No. 7-110598, there are described magnetite particles on the surface of which a co-precipitated product of silica and alumina is deposited.
At present, it has been strongly required to provide black magnetic iron oxide particles exhibiting a high electric resistance in a high voltage range, a low moisture absorption and an excellent dispersibility. However, such black magnetic iron oxide particles capable of satisfying these requirements have not been obtained until now.
That is, in the conventional techniques described in Japanese Patent Application Laid-open (KOKAI) Nos. 2002-72545 and 2005-289673, the electric resistance value of the particles has been noticed. However, the electric field applied to the toner particles within a printer in which the toner particles are actually used, is generally a high electric field though it varies depending upon the kind of printer used. As described in the below-mentioned Comparative Examples, these techniques may still fail to attain a sufficient electric resistance in such a high electric field.
In the conventional technique described in Japanese Patent Application Laid-open (KOKAI) No. 2007-314412, the electric resistance in a high voltage range has been noticed. However, as described in the below-mentioned Comparative Examples, the above technique may also still fail to attain a sufficient electric resistance in such a high voltage range.
The technique described in Japanese Patent Application Laid-Open (KOKAI) No. 7-110598 aims at obtaining magnetite particles having not only an excellent fluidity and a low oil absorption but also an excellent charging stability. However, the conventional technique may also fail to provide magnetite particles exhibiting a sufficiently high electric resistance in a high voltage range.
In order to achieve a high image density and a good keeping property of the high image density, it is important that a pigment used in the toner exhibits a high electric resistance value, and the electric resistance value of the pigment is also kept high even in a high voltage range. More specifically, even though the pigment exhibits a high electric resistance value in a low voltage range, if the electric resistance value is low in an electric field actually used, an electrostatic charge present on the surface of the toner tends to leak out through the pigment exposed to the surface of the toner as a leak site, resulting in low charge amount on the toner and, therefore, considerable deterioration in image density.
Thus, the conventional black magnetic iron oxide particles described in the above patent publications all have failed to satisfy the requirement of enhancing the electric resistance value in a high voltage range which has been strongly needed at the present time.
On the other hand, the conventional electrophotographic developing methods tend to suffer from the following problems.
As is well known in the art, in electrophotographic developing methods, there has been generally used a photosensitive member comprising a photoconductive material such as selenium, OPC (organic semiconductor), α-Si or the like, in which an electrostatic latent image is formed on the photosensitive member by various means, and then by using a magnetic brush method or the like, a toner having a polarity reverse to that of the latent image is attached thereon by an electrostatic force to form the developed image.
In the developing step of the above methods, there is used a developer comprising a toner and a carrier. The support particles called a carrier serve for imparting an appropriate positive or negative electrical quantity to the toner by frictional electrification, and transferring the toner to a developing zone near the surface of the photosensitive member on which a latent image is formed, through a developing sleeve in which magnets are accommodated, using a magnetic force thereof.
In recent years, the electrophotographic developing method has been widely applied to copying machines or printers. In these apparatuses, it has been demanded to meet various requirements including not only reproduction of thin lines, small characters, photographs, color originals or the like. With the development of copying machines and printers having a higher performance, a higher image quality and a higher copying or printing speed, it has been required to improve various properties of a developer used therein.
As is well known in the art, the developer has also been required to have such a durability that electric properties of the toner and the carrier are not significantly changed during use. For example, there tends to be caused such an undesirable phenomenon that a toner is firmly deposited onto the surface of the carrier particles, so that the charging property inherent to the carrier particles is lost (i.e., a so-called spent toner), or such a phenomenon that a resin coating layer formed on the surface of the respective carrier particles is peeled off with the passage of time, so that leak sites are formed thereon, thereby failing to appropriately charge the toner.
The carrier is required to have a certain suitable electric resistance value ranging from about 1×108 to 1×1016 Ω·cm. More specifically, when the carrier has an electric resistance value as low as 1×106 Ω·cm like iron powder carrier, there tend to arise the problems such as attachment of the carrier to image-bearing portions of a photosensitive member owing to injection of electric charges from a sleeve, and occurrence of defective latent images or lack of obtained images owing to escape of latent image-forming charges through the carrier. On the other hand, when the thickness of the insulating resin coating layer is increased, the electric resistance value of the carrier tends to become too high, so that charges on the carrier tend to hardly leak out, and the toner has a very high charge amount. As a result, although the image having a sharp edge is obtained, there tends to arise such a problem that the image having a large area shows a considerably low image density at a central portion thereof.
When the electric resistance value of the carrier has a large voltage dependency, the obtained image tends to generally has no gradation, so that even when using the carrier for a developer in copying machines or printers, it may be difficult to obtain images having a high image quality, and the applications thereof tend to be limited.
The iron powder carrier or ferrite carrier is usually used in the form of resin-coated particles. However, since the iron powder carrier has a true density as large as 7 to 8 g/cm3 whereas the ferrite carrier has a true density as large as 4.5 to 5.5 g/cm3, a large driving force is required for stirring these carriers in a developing device, resulting in significant mechanical damage to the device, occurrence of spent toner as well as deterioration of the charging property itself and facilitated damage to the photosensitive member. Further, since the adhesion between the surface of the iron powder carrier or ferrite carrier and the coating resin is not good, the coating resin tends to be gradually peeled off during use with the time, thereby causing variation in the charging property. As a result, the problems such as formation of defective images and adhesion of the carrier to the images tend to be caused.
The carrier of a magnetic material-dispersed type comprising spherical magnetic composite particles formed from magnetic iron oxide particles and a phenol resin as described in Japanese Patent Application Laid-Open (KOKAI) No. 2-220068 has a small true density as compared to the iron powder carrier or ferrite carrier and, therefore, exhibits an excellent durability against peeling of the coating resin owing to a less amount of energy upon impingement between the toner and carrier.
However, the carrier of a magnetic material-dispersed type has a low electric resistance value, and the electric resistance value exhibits a large voltage dependency. In addition, even though the carrier is coated with various resins to improve the electric resistance, when the resin-coated carrier is actually subjected to printing operation in recent copying machines and printers having such a tendency toward high copying or printing speed, high performance and high image quality, and further when the resin coating layer thereof suffers from abrasion, etc., there tend to arise the problems such as leak of electric charges upon development and poor gradation of obtained images owing to a large voltage dependency of the electric resistance value.
In particular, in recent years, the developer tends to be required to show a good durability over a whole service life of maintenance-free machines. Therefore, it is strongly required that the carrier of a magnetic material-dispersed type has a sufficient electric resistance, and the electric resistance of the magnetic carrier has a less voltage dependency.
Hitherto, as to the carrier of a magnetic material-dispersed type, there are known the technique of controlling an electric resistance value of the carrier by coating a surface of the respective spherical magnetic composite particles with a melamine resin (Japanese Patent Application Laid-Open (KOKAI) No. 3-192268); the technique of controlling an electric resistance value of the carrier by forming a coating layer comprising a cured copolymer resin obtained from one or more kinds of resins and a phenol resin on the surface of the respective spherical magnetic composite particles (Japanese Patent Application Laid-Open (KOKAI) No. 9-311505); the technique of controlling an electric resistance value of a carrier by incorporating a non-magnetic iron compound in the surface of respective spherical magnetic composite particles (Japanese Patent Application Laid-Open (KOKAI) Nos. 8-6303 and 2003-295523); carriers comprising magnetic iron oxide on the surface of which composite iron oxide of aluminum and iron is present (Japanese Patent Application Laid-Open (KOKAI) Nos. 2002-72545 and 2008-90012); etc.
In the techniques described in Japanese Patent Application Laid-Open (KOKAI) Nos. 3-192268 and 9-311505, the electric resistance values of these carriers are increased. However, since the carriers used in these techniques are not in the form of ferromagnetic compound particles whose surface is coated with an Al compound, the electric resistance thereof tends to be considerably lowered when a high voltage is applied thereto, i.e., tends to have a large voltage dependency.
Also, in the techniques described in Japanese Patent Application Laid-Open (KOKAI) Nos. 8-6303 and 2003-295523, it is possible to obtain a carrier having a high electric resistance value. However, since magnetic iron oxide particles whose surface is coated with an Al compound are not used as the ferromagnetic compound, the electric resistance value of the carrier tends to have a large voltage dependency.
In addition, in the techniques described in Japanese Patent Application Laid-Open (KOKAI) Nos. 2002-72545 and 2008-90012, it is possible to increase the electric resistance value of the carriers to some extent. However, as shown in the below-mentioned Comparative Examples, the carriers may fail to exhibit a sufficiently high electric resistance value.